toxoplasma gondii sea otters

How a Parasite in Cats is Killing Sea Otters

The Journey of Toxoplasma Gondii

How is it that a parasite transmitted by cats on land can contribute to the death of sea otters in the ocean?

The answer lies in a parasite called Toxoplasma gondii and the journey it takes from cats, through the environment, and then to wild marine mammals. It’s a story worthy of our attention, because humans play an active role in the process, and have the potential to fall ill to the same parasite.

Here you see Toxoplasma gondii's complex journey from prey species to cats to the sea (or to humans or livestock). Oocysts are key ingredient in that transport.

Toxoplasma gondii is a parasite that can infect virtually all warm-blooded animals, but the only known definitive hosts are cats – domesticated and feral house cats included. Cats catch the parasite through hunting rodents and birds and they offload it into the environment through their feces.

Between 15-18 percent of the U.S. human population actually carries Toxoplasma. The human body does not fight it off, so the parasite remains dormant in muscle or nervous tissue, usually causing little to no harm. That is unless your body is immunosuppressed in some way, in which case the parasite can become reactivated, leading to illness and even death. This has been the unfortunate case for many AIDS patients.

Oocysts are egg-like structures that are released by cats in their feces – formed from the sexual multiplication of the parasite, which only occurs in the gut of the definitive host. Once infected, cats can release hundreds of millions of Toxoplasma oocysts in their feces, for up to two weeks.

Dr. Karen Shapiro working in the Monterey Bay.

Pregnant women are often told to avoid contact with cat litter for this reason – a mother who is infected during pregnancy can pass the infection to her unborn child, which can result in devastating disease in the fetus.

Oocysts are robustly resistant to environmental insults, which renders Toxoplasma stable in cold water for years. When the oocysts reach the ocean, they attach to kelp and marine snow in the seawater, at which point they can incorporate into the marine food web. Sticky substances excreted by seaweed allow the oocysts to directly adhere to kelp, the preferred food of turban snails. Oocysts are then accidentally ingested by snails, which are subsequently preyed upon by the sea otters. The journey from cat to sea otter is complete.

Otters can also ingest oocysts through grooming and other prey items. Scientists recently detected Toxoplasma in green-lipped mussels from New Zeland intended for human consumption.

How can you prevent it?

There are three main factors that can contribute to Toxoplasma infections in sea otters:

Loss of wetlands: The destruction of wetlands leads to the loss of filtering capacity, which could otherwise keep fecal pathogens from running into the sea.

Water runoff: Our cities are built on impervious surfaces; rain that falls on cement creates more runoff than rain that falls on natural earth, which contributes to increased runoff that can carry fecal pathogens to the sea.

With those risk factors in mind, what can people do to alleviate the problem?

Keep cats indoors, and if you do allow your cat outdoors, it’s a good idea to have litter boxes both inside and out. That way feces never actually enters the environment. If your cat does poop outdoors, collect and dispose of it by placing it in a plastic bag before putting it in the trash.

Spay or neuter your pet cat. A lower domestic cat population means that there are fewer cats roaming around to catch and spread Toxoplasma.

Support wetland conservation and reconstruction, as well as other natural lands that border our oceans.

Think green when designing your home or landscaping your yards. Leaving as much natural cover as possible will help rainwater trickle back into aquifers instead of generating runoff.

A Cautionary Tale

No problem occurs in isolation. The contaminants that humans and their domesticated animals contribute to the environment can have a lasting effect that stretches beyond their backyard. In that sense, Toxoplasma is probably only the tip of the iceberg. If a cat-derived parasite can sicken or kill sea otters, what other pathogens are moving through the environment and into the ocean? And what health effects are they having, now or in the future, on marine animals and people?

The One Health approach, in which animals, people and the environment are all studied in confluence, will prove crucial in answering these challenging questions.